An optical transmitter used in an optical communication system is generally provided with an optical modulator for modulating carrier light depending on transmission data. A well-known embodiment of an optical modulator has a configuration including a Mach-Zehnder modulator.
FIG. 1 illustrates a configuration of one example of a Mach-Zehnder modulator. In FIG. 1, an optical waveguide (a splitting section 101, a pair of optical waveguides 102a and 102b, and a combining section 103) is formed on a substrate 100. The substrate 100 has an electro-optical effect. The splitting section 101 splits input light and guides the light to the optical waveguides 102a and 102b. In this example, the input light is CW light of a TM mode. The optical waveguides 102a and 102b propagate the input light, respectively. The combining section 103 combines optical signals propagated through the optical waveguides 102a and 102b. A signal electrode 104 is formed on one of the optical waveguides 102a and 102b (the optical waveguide 102b in the example illustrated in FIG. 1). A DC electrode 105 is also formed on one of the optical waveguides 102a and 102b (the optical waveguide 102b in the example illustrated in FIG. 1).
In the Mach-Zehnder modulator with the configuration above, when a data signal is provided for the signal electrode 104, the refractive index of the optical waveguide is controlled based on the data signal, and the phase difference between the optical waveguides 102a and 102b is changed by Mach-Zehnder interference. Therefore, an optical signal modulated depending on the data signal is generated. By providing DC bias voltage through the DC electrode 105, the bias point of the Mach-Zehnder modulator is adjusted. The DC bias voltage is adjusted by feedback control so that, for example, the output power is minimized when the data signal is OFF.
Recently, a transmitting system using a plurality of optical modulators has been implemented for large capacity optical signals. For example, multi-level modulation such as QPSK (including DQPSK) etc., polarization multiplexing, etc. has been proposed.
A polarization multiplexing modulator includes, for example as illustrated in FIG. 2, a pair of Mach-Zehnder modulators 110A and 110B, and a polarization beam coupler (PBC) 111 for polarization multiplexing the output signals of the Mach-Zehnder modulators 110A and 110B. Each of the Mach-Zehnder modulators 110A and 110B may be realized by the Mach-Zehnder modulator illustrated in FIG. 1 in this example. The polarization beam coupler 111 multiplexes polarizations orthogonal to each other (TE mode, TM mode).
However, with this configuration, the size of the optical transmitter increases. That is, to reduce the size of the optical transmitter, it is desired that a plurality of optical modulators are integrated on one chip.
FIG. 3 illustrates configuration of an optical device having a plurality of optical modulators on one chip. The optical device includes a splitting waveguide 120 and Mach-Zehnder modulators A and B on the substrate 100. The splitting waveguide 120 splits the input CW light and guides the resultant light to the Mach-Zehnder modulators A and B. The configuration and operation of each of the Mach-Zehnder modulators A and B are similar to the one described above with reference to FIG. 1. However, a data signal RF-A is provided for the Mach-Zehnder modulator A using a signal electrode 104A, and a data signal RF-B is provided for the Mach-Zehnder modulator B using a signal electrode 104B. Therefore, the optical device generates modulated optical signal A corresponding to the data signal RF-A and modulated optical signal B corresponding to the data signal RF-B. The modulated optical signal A and the modulated optical signal B are multiplexed by the polarization beam coupler 111.
The optical device provided with a plurality of Mach-Zehnder modulators is described, for example, in the Japanese Laid-open Patent Publication No. 2008-116865 or the Japanese Laid-open Patent Publication No. 2007-57785.
In the optical device provided with a plurality of optical modulators on one chip, it is often requested that an input ports for inputting data signals to the optical modulators are arranged on one side of the chip. However, an optical connector for inputting data signal normally has a size of several millimeters. Therefore, the interval S illustrated in FIG. 3 cannot be smaller than the optical connector.
On the other hand, in order to reduce the drive voltage of each optical modulator, it is desired to have a longer interaction length (length of an optical waveguide whose characteristic is controlled by providing a signal with an electrode). However, to have a longer interaction length of the Mach-Zehnder modulator B with the configuration illustrated in FIG. 3, the signal electrode 104B for providing the data signal RF-B is formed to be extended from the input port (RF-B) toward the input side of the modulator. That is, the signal electrode 104B is excessively longer by the length of Lf. When the signal electrode is longer, the high frequency component of the data signal is attenuated, and the modulation band is degraded. That is, there is the possibility that the waveform of the modulation signal is degraded, and the communication quality is reduced. If the substrate 100 is longer, it is possible to have a shorter path length of the signal electrode while maintaining the interval between the input ports of the electric signals. However, the configuration does not satisfy the request of downsizing the optical device.
As described above, it is difficult to maintain the quality of modulated signal of an optical device provided with a plurality of optical modulators.